The invention relates to a discharge device, especially for a gas laser, having a discharge chamber through which a gas flows at high velocity and in which the gas discharge burns as a directcurrent discharge between an anode and a cathode situated opposite the anode, the anode and cathode serving as main electrodes extending transversely of the gas stream. Such a discharge device also has at least one pre-ionizing electrode which is enveloped in dielectric material and fed with alternating current, has a length corresponding approximately to that of the cathode and anode, and is situated ahead of the cathode and/or anode in the direction of flow of the gas.
A discharge device of this kind, in the form of a gas laser, is disclosed in U.S. Pat. No. 4,488,309. This gas laser has an anode extending in the direction of flow and defining one side of the gas discharge channel carrying the gas; opposite this anode is a plurality of pin-like single cathodes each disposed transversely of the gas flow and connected by high-ohmic input resistances to the direct-current source. Approximately in the center between the anode and the single cathodes there is disposed a pre-ionizing electrode in rod form having a conductive core contained inside of a dielectric coating. On account of the pre-ionizing electrode disposed in the center of the gas channel undesired turbulence is produced resulting in flow losses. The pre-ionizing electrode is connected to an alternating current source. Furthermore, a capacitance is present between the anode and the ground potential, and an inductance between the output of the power source and the anode, and these serve for the accumulation of energy for pulsing. The capacitor and inductor must be designed for a high pulse energy in order to keep the constant power level of the direct-current source low. With this arrangement a stable DC luminous discharge is produced, which can easily be switched and pulsed, i.e., has good starting characteristics; also, the voltage of the DC discharge is lowered. In this circuit the pre-ionizing discharge burns exclusively between the pre-ionizing electrode and the anode, since high input resistances are in the cathode circuit. The result is that the gas volume in the discharge chamber is irregularly pre-ionized and the power required for the pre-ionization is relatively high. The cost of the power supply circuit is high, since the production of high-frequency alternating-current power is disproportionately great in comparison to a DC power supply circuit.
Federal Patent 28 56 328, to which U.S. Pat. No. 4,449,220 corresponds (incorporated herein by reference), discloses another discharge device for a gaseous medium, in which the auxiliary discharge is a direct-current pre-ionization produced by a plurality of pre-ionizing electrodes in the form of pins projecting from the wall of the discharge chamber. A power of the order of magnitude of 40% of the total input power is required for the auxiliary discharge. This is important also because the individual auxiliary electrodes are provided with high input impedances to stabilize the additional discharge, and they cause considerable additional loss of power. Additional cost is involved if the discharge is to be switched rapidly, since either two independent supply voltages must be used, or the main and secondary discharges must be clearly separated from one another.
One embodiment of pre-ionizing devices of this kind is disclosed in WO 82/01281, which especially describes the shape of single additional electrodes for the direct-current pre-ionization, and gives disks, T-shaped hooks, U-shaped hooks and L-shaped hooks as possible shapes, extending into the discharge chamber. This pre-ionizing arrangement calls for a high power input.